Today, telephones having a speakerphone allowing for hands-free operation or for a number of people in a room to simultaneously communicate on the telephone are being used with increasing frequency. Almost every modern office telephone includes a speakerphone to pickup and transmit sounds without having to use the telephone handset. Such speakerphones allow a number of people to simultaneously converse and interact over a single telephone call.
One problem associated with speakerphones is that the people involved in the conversation will most likely be different distances from the microphone that receives their voices. As a result, the volume of the different speaker's voices will vary with their distance from the microphone. For example, the volume of speakers physically situated near the microphone tends to be loud, while other speakers are barely audible. Simply amplifying the microphone so that the lowest volume speakers can be heard may result in other speaker's voices being over-amplified. This effect can be very distracting, making it difficult to listen and understand the conversation.
In a speakerphone application, automatic gain control (“AGC”) circuits can be used to automatically vary the amplification of different speaker's voices to equalize their relative volumes. AGC circuitry is typically employed to maintain a constant amplification gain for electronic circuitry that may otherwise vary over time. Generally, the gain of an electronic circuit may change over time, varying with several factors such as the amplitude or frequency of the input signal, the ambient temperature, as well as other external and internal factors.
A conventional digital AGC implementation utilizes a linear negative feedback loop. Typically, the AGC circuitry amplifies an input signal to provide an output signal level. The AGC circuitry varies the amplification gain to provide the desired output signal level. To determined the appropriate amplification gain, the output signal level is compared to a desired reference signal level and an error signal computed as the difference between the output signal level and the desired reference signal level. The error signal is fed back to adjust the amplification gain such that the output signal level matches the desired reference signal level. Typically, amplification gain adjustments are made proportional to the error signal computed with respect to an estimate of the output power. The output power is determined as an exponentially weighed average over a time constant.
This conventional type of AGC feedback system does not handle voice signals particularly well because it must deal with transitions from a period of significant signal level, such as a voice signal, to a non-signal, such as a period of silence after the transition has already occurred. This conventional system assumes the input is statistically constant, or in other words, wide-sense stationary. An actual voice signal, however, is not constant but varies widely during speech as well as in between periods of speech and non-speech. An automatic gain control employing only feedback compensation may therefore cause the boundaries between intervals of speech and non-speech to become blurred.